Hemant Chitale Simple ExplainPlan SG RACSIG 12Oct11

Explain Plan : Execution Plan

Hemant K Chitale

SG RACSIG 12-Oct-11

Introduction

The command EXPLAIN PLAN is an instruction to *present* the Execution Plan.

The Execution Plan is normally determined at RunTime based on : Presence of the same SQL, with Execution Plan, in

the Shared Pool

Bind Peeking

Hard Parse (note : tkprof and autotrace EXPLAIN do Hard Parses)

Methods

Simplest command : EXPLAIN PLAN FOR select . ;

SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY));

Plan for existing SQL already parsed and executed SELECT * FROM

TABLE(DBMS_XPLAN.DISPLAY_CURSOR(&sql_id,&child_number));

Plan from AWR SELECT * FROM

TABLE(DBMS_XPLAN.DISPLAY_AWR(&sql_id));

Example -- 1

EXPLAIN PLAN shows

Join Method

Expected Cardinality

Indexes

Access and Filter Predicates

Whether Dynamic Sampling is used

SQL> l

1 explain plan for select c.cust_id, c.cust_name, f.prod_id, f.time_id, f.quantity_sold

2 from sales_fact f, customer_dim c

3 where f.cust_id = c.cust_id

4* order by c.cust_id

SQL> /

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT

----------------------------------------------------------------------------------------------------

Plan hash value: 3912614538

--------------------------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |

--------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 966K| 56M| | 6858 (2)| 00:01:23 |

| 1 | MERGE JOIN | | 966K| 56M| | 6858 (2)| 00:01:23 |

| 2 | SORT JOIN | | 7426 | 304K| 776K| 102 (1)| 00:00:02 |

| 3 | TABLE ACCESS FULL| CUSTOMER_DIM | 7426 | 304K| | 19 (0)| 00:00:01 |

|* 4 | SORT JOIN | | 966K| 17M| 59M| 6756 (2)| 00:01:22 |

| 5 | TABLE ACCESS FULL| SALES_FACT | 966K| 17M| | 1038 (2)| 00:00:13 |

--------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

4 - access("F"."CUST_ID"="C"."CUST_ID")

filter("F"."CUST_ID"="C"."CUST_ID")

Note

-----

- dynamic sampling used for this statement

22 rows selected.

SQL>

SQL> select count(*) from sales_fact;

COUNT(*)

----------

959040

SQL> select count(*) from customer_dim;

COUNT(*)

----------

7005

SQL>

Observations -- 1

Can you identify which table / index it used Dynamic Sampling on ?

Why are there two SORT JOIN and one MERGE JOIN operations ?

More Info: From an Event 10053 trace ***************************************

BASE STATISTICAL INFORMATION

***********************

Table Stats::

Table: CUSTOMER_DIM Alias: C (NOT ANALYZED)

#Rows: 6535 #Blks: 80 AvgRowLen: 100.00

Column (#1): CUST_ID(NUMBER) NO STATISTICS (using defaults)

AvgLen: 22.00 NDV: 204 Nulls: 0 Density: 0.0048967

Index Stats::

Index: CUST_ID_PK Col#: 1

LVLS: 1 #LB: 14 #DK: 7005 LB/K: 1.00 DB/K: 1.00 CLUF: 6332.00

***********************

Table Stats::

Table: SALES_FACT Alias: F

#Rows: 966158 #Blks: 4655 AvgRowLen: 29.00

Column (#2): CUST_ID(NUMBER)

AvgLen: 5.00 NDV: 4893 Nulls: 0 Density: 2.0437e-04 Min: 2 Max: 100970

Index Stats::

Index: SALES_CUST_BMP_NDX Col#: 2

LVLS: 1 #LB: 358 #DK: 7005 LB/K: 1.00 DB/K: 1.00 CLUF: 7005.00

***************************************

There are statistics on the CUST_ID_PK index but not on the CUSTOMER_DIM table and CUST_ID column. (CREATE INDEX had included COMPUTE STATISTICS)

Example -- 2

How does Explain Plan handle BINDs ?

It doesnt ! Explain Plan does *not* peek binds.

See http://tkyte.blogspot.com/2007/04/when-explanation-doesn-sound-quite.html

Explain plan is blind to the bind

SQL> create table Explain_Binds (owner varchar2(30) not null, object_name varchar2(30));

Table created.

SQL> insert into Explain_Binds select owner, object_name from dba_objects ;

46170 rows created.

SQL> create index Explain_Binds_NDX_1 on Explain_Binds(owner);

Index created.

SQL>

SQL> select owner, count(*) from dba_objects group by owner order by owner ;

OWNER COUNT(*)

------------------------------ ----------

DBSNMP 46

EXFSYS 281

HEMANT 135

ORDPLUGINS 10

ORDSYS 1720

OUTLN 8

PUBLIC 18548

SH 296

SI_INFORMTN_SCHEMA 8

SYS 22432

SYSMAN 1321

SYSTEM 449

TSMSYS 3

WMSYS 242

XDB 672

15 rows selected.

SQL> exec dbms_stats.gather_table_stats('','EXPLAIN_BINDS',-

> estimate_percent=>100,method_opt=>'FOR COLUMNS OWNER SIZE 250');

PL/SQL procedure successfully completed.

SQL>

SQL> -- explain for Literal

SQL> explain plan for select object_name from explain_binds where owner = 'TSMSYS';

Explained.


PLAN_TABLE_OUTPUT

----------------------------------------------------------------------------------------------------


---------------------------------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

---------------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 3 | 69 | 2 (0)| 00:00:01 |

| 1 | TABLE ACCESS BY INDEX ROWID| EXPLAIN_BINDS | 3 | 69 | 2 (0)| 00:00:01 |

|* 2 | INDEX RANGE SCAN | EXPLAIN_BINDS_NDX_1 | 3 | | 1 (0)| 00:00:01 |

---------------------------------------------------------------------------------------------------


---------------------------------------------------

2 - access("OWNER"='TSMSYS')

14 rows selected.

SQL>

SQL> explain plan for select object_name from explain_binds where owner = 'SYS';

Explained.


PLAN_TABLE_OUTPUT

----------------------------------------------------------------------------------------------------


-----------------------------------------------------------------------------------


-----------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 22432 | 503K| 56 (2)| 00:00:01 |

|* 1 | TABLE ACCESS FULL| EXPLAIN_BINDS | 22432 | 503K| 56 (2)| 00:00:01 |

-----------------------------------------------------------------------------------


---------------------------------------------------

1 - filter("OWNER"='SYS')

13 rows selected.

SQL>

SQL> -- explain for Bind

SQL> variable ow varchar2(30);

SQL> exec :ow := 'TSMSYS';


SQL> explain plan for select object_name from explain_binds where owner = :ow;

Explained.


PLAN_TABLE_OUTPUT

----------------------------------------------------------------------------------------------------


---------------------------------------------------------------------------------------------------


---------------------------------------------------------------------------------------------------




---------------------------------------------------------------------------------------------------


---------------------------------------------------

2 - access("OWNER"=:OW)

14 rows selected.

SQL>

SQL> exec :ow := 'SYS';


SQL> explain plan for select object_name from explain_binds where owner = :ow;

Explained.


PLAN_TABLE_OUTPUT

----------------------------------------------------------------------------------------------------


---------------------------------------------------------------------------------------------------


---------------------------------------------------------------------------------------------------




---------------------------------------------------------------------------------------------------


---------------------------------------------------

2 - access("OWNER"=:OW)

14 rows selected.

SQL>

Observations -- 2

Explain Plan with Bind assumed the same Cardinality for different Binds

It did a Hard Parse without peeking the Bind

Other Methods -- 1 SQL> l

1* select * from table(dbms_xplan.display_cursor('&sql_id'))

SQL> /

Enter value for sql_id: 1qgc21cjkb741

PLAN_TABLE_OUTPUT

------------------------------------------------------------------------------------------------------------------------------------

SQL_ID 1qgc21cjkb741, child number 0

-------------------------------------

select c.cust_id, c.cust_name, f.prod_id, f.time_id, f.quantity_sold from sales_fact f,

customer_dim c where f.cust_id = c.cust_id and f.cust_id = 2 order by f.prod_id, f.time_id


-----------------------------------------------------------------------------------------------------


-----------------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | | | 45 (100)| |

| 1 | SORT ORDER BY | | 197 | 12017 | 45 (3)| 00:00:01 |

|* 2 | HASH JOIN | | 197 | 12017 | 44 (0)| 00:00:01 |

| 3 | TABLE ACCESS BY INDEX ROWID | CUSTOMER_DIM | 1 | 42 | 2 (0)| 00:00:01 |

|* 4 | INDEX UNIQUE SCAN | CUST_ID_PK | 1 | | 1 (0)| 00:00:01 |

| 5 | TABLE ACCESS BY INDEX ROWID | SALES_FACT | 197 | 3743 | 42 (0)| 00:00:01 |

| 6 | BITMAP CONVERSION TO ROWIDS| | | | | |

|* 7 | BITMAP INDEX SINGLE VALUE | SALES_CUST_BMP_NDX | | | | |

-----------------------------------------------------------------------------------------------------


---------------------------------------------------


4 - access("C"."CUST_ID"=2)

7 - access("F"."CUST_ID"=2)

27 rows selected.

SQL>

Other Methods -- 2 SQL> select * from table(dbms_xplan.display_awr('&sql_id'));


PLAN_TABLE_OUTPUT

------------------------------------------------------------------------------------------------------------------------------------

SQL_ID 1qgc21cjkb741

--------------------

select c.cust_id, c.cust_name, f.prod_id, f.time_id, f.quantity_sold from sales_fact f,



-----------------------------------------------------------------------------------------------------


-----------------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | | | 45 (100)| |

| 1 | SORT ORDER BY | | 197 | 12017 | 45 (3)| 00:00:01 |

| 2 | HASH JOIN | | 197 | 12017 | 44 (0)| 00:00:01 |

| 3 | TABLE ACCESS BY INDEX ROWID | CUSTOMER_DIM | 1 | 42 | 2 (0)| 00:00:01 |

| 4 | INDEX UNIQUE SCAN | CUST_ID_PK | 1 | | 1 (0)| 00:00:01 |

| 5 | TABLE ACCESS BY INDEX ROWID | SALES_FACT | 197 | 3743 | 42 (0)| 00:00:01 |

| 6 | BITMAP CONVERSION TO ROWIDS| | | | | |

| 7 | BITMAP INDEX SINGLE VALUE | SALES_CUST_BMP_NDX | | | | |

-----------------------------------------------------------------------------------------------------

20 rows selected.

SQL>

Unfortunately, the PREDICATES section is missing. (did you notice the transitive closure ?)

Other Methods -- 3

SQL> set serveroutput off

SQL> select /*+ gather_plan_statistics */

c.cust_id, c.cust_name, f.prod_id, f.time_id, f.quantity_sold

from sales_fact f, customer_dim c

where f.cust_id = c.cust_id

and f.cust_id = 2

order by f.prod_id, f.time_id

/

178 rows selected.

SQL>

SQL> select * from table(dbms_xplan.display_cursor('','','ALLSTATS LAST'));

PLAN_TABLE_OUTPUT

----------------------------------------------------------------------------------------------------------------------------------

--

SQL_ID 36cf1j4n2wcw3, child number 0

-------------------------------------

select /*+ gather_plan_statistics */ c.cust_id, c.cust_name, f.prod_id, f.time_id, f.quantity_sold from sales_fact f,



----------------------------------------------------------------------------------------------------------------------------------

--

------

| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem |

Used

-Mem |

----------------------------------------------------------------------------------------------------------------------------------

--

------

| 1 | SORT ORDER BY | | 1 | 197 | 178 |00:00:00.01 | 157 | 13312 | 13312

|12288

(0)|

|* 2 | HASH JOIN | | 1 | 197 | 178 |00:00:00.01 | 157 | 788K| 788K|

306

K (0)|

| 3 | TABLE ACCESS BY INDEX ROWID | CUSTOMER_DIM | 1 | 1 | 1 |00:00:00.01 | 4 | | |

|

|* 4 | INDEX UNIQUE SCAN | CUST_ID_PK | 1 | 1 | 1 |00:00:00.01 | 2 | | |

|

| 5 | TABLE ACCESS BY INDEX ROWID | SALES_FACT | 1 | 197 | 178 |00:00:00.01 | 153 | | |

|

| 6 | BITMAP CONVERSION TO ROWIDS| | 1 | | 178 |00:00:00.01 | 2 | | |

|

|* 7 | BITMAP INDEX SINGLE VALUE | SALES_CUST_BMP_NDX | 1 | | 1 |00:00:00.01 | 2 | | |

|

----------------------------------------------------------------------------------------------------------------------------------

--

------


---------------------------------------------------


4 - access("C"."CUST_ID"=2)

7 - access("F"."CUST_ID"=2)

26 rows selected.

SQL>

More Options

For more options to DBMS_XPLAN see $ORACLE_HOME/rdbms/admin/dbmsxpln.sql

More examples

From https://forums.oracle.com/forums/thread.jspa?messageID=9881406

SELECT Max(logId) AS logId FROM online_users_t

WHERE online_users_date >= to_date('2011-09-19 10:00:00') - 3.2 AND online_users_date =TO_DATE(' 2011-09-16 05:12:00', 'syyyy-mm-dd

hh24:mi:ss') AND "ONLINE_USERS_DATE"

11g

Real Time monitoring using V$SQL_MONITOR and V$SQL_PLAN_MONITOR

Quiz

Whats the difference between Index Unique Scan and Index Range Scan when the target *is* a Unique Index ?

Whats the difference between Index Full Scan and Index Fast Full Scan ?

What are the number of rows when a BitMap index appears ?

What is View Merging ? What is SubQuery UnNesting ?

Documents

Hemant Chitale Simple ExplainPlan SG RACSIG 12Oct11